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Constraints on the petrologic structure of the subduction zone slab-mantle interface from Franciscan Complex exotic ultramafic blocks

King, Robert L. and Kohn, Matthew J. and Eiler, John M. (2003) Constraints on the petrologic structure of the subduction zone slab-mantle interface from Franciscan Complex exotic ultramafic blocks. Geological Society of America Bulletin, 115 (9). pp. 1097-1109. ISSN 0016-7606. doi:10.1130/B25255.1.

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Ultramafic blocks within mud-matrix mélange of the Franciscan Complex, California, preserve a series of metasomatic mineral zones generated by infiltration of Si-rich hydrous fluids during subduction. We describe the petrology and geochemistry of the metasomatic zones and compare them to current model predictions for the metasomatism of the mantle wedge by subduction zone fluids. Fluid flow affected a Cr-spinel lherzolite protolith to form first serpentinite, then a talc-dominated rock, and finally an amphibole-rich assemblage. A diverse suite of accessory minerals in the amphibole-rich zone (titanite + clinozoisite + zircon + apatite) suggests that the trace element signature of subduction zone fluids may be fractionated in this zone. Oxygen isotopic evidence suggests that the ultramafic blocks equilibrated with metasomatic fluids during serpentinization and that subsequent reactions occurred in equilibrium with these fluids in a temperature range of 450–500 °C. Whole-rock geochemistry indicates mobility of many elements into and out of the blocks during metasomatism, including elements such as Ti which are currently considered to have low solubilities in such fluids. Taken as a whole, the blocks appear to preserve the metasomatic structure of the slab-mantle interface in subduction zones and imply that the chemistry of slab-derived fluids is modified as they pass through these metasomatic zones in the mantle wedge. Our results suggest that the primary composition of subduction zone fluids is not likely reflected by arc magmas. Instead, we propose that arc magmas are derived from regions of the mantle fluxed by fluids residual to the metasomatic processes we observe.

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Additional Information:© 2003 Geological Society of America. Manuscript received by the Society: 10 September 2002; revised manuscript received: 9 February 2003; manuscript accepted: 4 March, 2003. This study presents the results of Robert L. King’s B.A. and M.S. theses. L. Reinen, C. Davidson, L. Bettison-Varga, R. Varga, J. Shervais, J. Knapp, C. Parkinson, A. Wilson, J. Ryan, G. Bebout, J. Wakabayashi, and C. Manning provided constructive discussions or reviews of earlier versions of this study. Formal reviews by J. Ayers, M. Cloos, and associate editor C. Miller substantially improved the clarity and presentation of the manuscript. Support was provided by the Keck Geology Consortium, the University of South Carolina, and National Science Foundation grant EAR0073803 (to Matthew J. Kohn).
Funding AgencyGrant Number
Keck Geology ConsortiumUNSPECIFIED
University of South CarolinaUNSPECIFIED
Subject Keywords:fluids; Franciscan Complex; mantle; metasomatism; subduction zones; ultramafic rocks
Issue or Number:9
Record Number:CaltechAUTHORS:20121024-152418331
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35078
Deposited On:25 Oct 2012 20:45
Last Modified:09 Nov 2021 23:12

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